We have no trio packaged yet.
https://github.com/python-trio/trio/issues/1437

--- anyio-4.2.0/src/anyio/_core/_eventloop.py.orig
+++ anyio-4.2.0/src/anyio/_core/_eventloop.py
@@ -19,7 +19,7 @@
     from ..abc import AsyncBackend
 
 # This must be updated when new backends are introduced
-BACKENDS = "asyncio", "trio"
+BACKENDS = "asyncio",
 
 T_Retval = TypeVar("T_Retval")
 PosArgsT = TypeVarTuple("PosArgsT")
--- anyio-4.2.0/pyproject.toml.orig
+++ anyio-4.2.0/pyproject.toml
@@ -43,7 +43,6 @@
 [project.optional-dependencies]
 trio = ["trio >= 0.23"]
 test = [
-    "anyio[trio]",
     "coverage[toml] >= 7",
     "exceptiongroup >= 1.2.0",
     "hypothesis >= 4.0",
--- anyio-4.2.0/tests/test_taskgroups.py.orig
+++ anyio-4.2.0/tests/test_taskgroups.py
@@ -67,7 +67,6 @@
     "module",
     [
         pytest.param(asyncio, id="asyncio"),
-        pytest.param(pytest.importorskip("trio"), id="trio"),
     ],
 )
 def test_run_natively(module: Any) -> None:
--- anyio-4.2.0/tests/conftest.py.orig
+++ anyio-4.2.0/tests/conftest.py
@@ -40,7 +40,6 @@
             marks=uvloop_marks,
             id="asyncio+uvloop",
         ),
-        pytest.param("trio"),
     ]
 )
 def anyio_backend(request: SubRequest) -> tuple[str, dict[str, Any]]: